1. Field of the Invention
This invention relates to the recovery of oil from subterranean reservoirs by chemical flooding methods.
2. Description of the Prior Art
Crude oil which has accumulated in subterranean reservoirs is recovered or produced through one or more wells drilled into the reservoir. Initial production of the crude oil is accomplished by "primary recovery" techniques wherein only the natural forces present in the reservoir are utilized to produce the oil. However, upon depletion of these natural forces and the termination of primary recovery a large portion of crude oil remains trapped within the reservoir. Also, many reservoirs lack sufficient natural forces to be produced by primary methods from the very beginning. Recognition of these facts has led to the development and use of many enhanced oil recovery techniques. Most of these techniques involve injection of at least one fluid into the reservoir to produce an additional amount of crude oil therefrom. Some of the more common methods are water-flooding, steam-flooding, miscible-flooding, CO.sub.2 -flooding, polymer-flooding, surfactant-flooding, caustic-flooding, and in situ combustion.
Water-flooding which involves injection of water into the subterranean oil reservoir for the purpose of displacing the crude oil from the pore spaces of the reservoir rock towards the producing wells is the most economical and widely used of enhanced oil recovery methods. Nevertheless, water does not displace oil with high efficiency because of the immiscibility of water and oil, and because of the high interfacial tension between them.
Surfactant-flooding involves the addition of one or more surface active agents or surfactants to the water-flood for the purpose of minimizing the water-flooding problems mentioned above. This has been an area of active interest in the art of enhanced oil recovery methods for many years. For example, in 1941, U.S. Pat. No. 2,233,381 disclosed the use of polyglycol ether as a surfactant that increases the capillary displacement efficiency of an aqueous flood. U.S. Pat. No. 3,302,713 discloses the use of petroleum sulfonates as effective surfactants in oil recovery operations. Other surfactants proposed for use in oil recovery processes include alkyl pyridinium salts, alkylsulfates, alkylarylsulfates, ethoxylated alkyl or alkylarylsulfates, alkylarylsulfonates, alkylsulfonates, alkylpolyethoxyalkylene sulfonates, alkylarylpolyethoxyalkylenesulfonates, and quaternary ammonium salts.
Any of the above surfactants may be effective under ideal conditions, however, there are problems concerned with the use of each in most petroleum reservoirs. One of the most serious problems arises from the adsorption of the surfactant chemicals onto the mineral grains of the reservoir rock resulting in an ever declining concentration of the materials as they progress through the reservoir. Compensation for this phenomenon has often required such large amounts of surfactant materials to be injected as to render the use of such materials uneconomical.
Another serious problem inherent in any flooding technique concerns the sweep efficiency of the flooding operation. The sweep efficiency for a flooding operation is directly related to the ratio of the relative mobilities of the displacing and displaced fluids. In almost all cases the unmodified surfactant fluid has a higher relative mobility than that of the residual oil in the reservoir. This results in poor sweep efficiency and is indicated by premature breakthrough of the injected surfactant fluid into the production wells. This premature breakthrough is caused by a selective fingering of the injected fluid through the reservoir leaving a substantial portion of the reservoir volume unswept by the injected surfactant fluid.
One solution to this problem of poor sweep efficiency is to increase the viscosity of the displacing fluid. The common method of achieving this has been to add organic polymers to the injected water which has the effect of increasing water viscosity thereby increasing the sweep efficiency of the recovery technique. U.S. Pat. No. 3,390,529 and U.S. Pat. No. 3,282,337, teach the use of aqueous polyacrylamide solutions to increase the viscosity of the injected water. Polysaccharides as taught in U.S. Pat. No. 3,581,824 have been used for the same purpose. Unfortunately, these polymers are expensive and are adsorbed on the mineral grains of the reservoir rock in much the same way as the surfactants are adsorbed as discussed hereinabove.
The above described problems have been recognized by those skilled in the art of oil recovery and certain so-called "sacrificial" compounds have been proposed to pretreat the formation in order to decrease the adsorption of subsequently injected surfactants and/or polymers. For example, U.S. Pat. No. 3,424,054 discloses the use of aqueous solutions of piperidine. U.S. Pat. No. 3,469,630 discloses the use of sodium carbonate and inorganic polyphosphates. U.S. Pat. No. 3,384,171 discloses the use of unmodified lignosulfonates. However, none of these materials have been entirely satisfactory from a standpoint of performance and economics.